Method and device for the continuous production of polyamide 6, 6 and copolyamides thereof

ABSTRACT

A method and device for the continuous production of copolyamide and polyamide, the starting materials for which comprise hexamethylenediamine adipate (nylon salt), water and, in the case of copolyamide, additionally lactam, comprise a first method step ( 2 ) which is carried out under pressure at a temperature of between 180 and 280° C. After the pass through the first method step, the prepolymer is subjected to a second method step ( 5 ), from which the evaporated water is removed or driven off by inert gas. The water evaporated off in the first method step ( 2 ) with the reaction components contained therein is fed to the at least one further method step ( 5 ) and the water is only driven off in said method step or subsequent method steps.

The invention relates to a method and a device for carrying out themethod for the continuous production of copolyamide or polyamide of thetype stated in the preamble of claim 1 and 6, respectively.

For the polymerization of the hexamethylenediamine salt of adipic acid(AH salt) or adipic acid and hexamethylenediamine, it is known that thestarting material used is an aqueous AH salt solution which is heated ina pressure-resistant reactor to a temperature in the range of 220-280°C., with the aim of preparing an AH precondensate in which the NH₂groups of the hexamethylenediamine are reacted with the COOH groups ofthe adipic acid.

It is known that the aqueous solution of the hexamethylenediamine saltof adipic acid is heated under a pressure which is lower than the vaporpressure generated by the solution, with simultaneous evaporation of thewater, and the most readily volatile, expelled hexamethylenediamine isretained in a column and is recycled to the polymerization process (U.S.Pat. No. 2,689,839; U.S. Pat. No. 3,960,820). When such a precondensateis let down, a small amount of hexamethylenediamine may escape. Forcompensating the viscosity-lowering excess of acid, hexamethylenediamineis added in excess (U.S. Pat. No. 3,193,535; DE 2 417 003). Dewateringand postcondensation of the polyamide melt are carried out by knownmethods.

DE-A1 196 21 088 furthermore discloses that the complete conversion ofthe NH₂ groups of the hexamethylenediamine and of the COOH groups of theadipic acid can be achieved by a closed procedure of the first methodstage in a polymerization reactor under the full pressure generated atthe corresponding reaction temperature. Here, however, a part of theevaporating water has to be separated off at high pressure and hightemperature in the polymerization reactor, which results in high energyconsumption without it being possible completely to avoid diaminelosses.

It was the object of the invention to provide a method and a device ofthe type stated at the outset, in which method and in which device thewater can be separated off at relatively low temperatures and pressuresand with reduced diamine losses.

This object is achieved by the features stated in patent claim 1 and 6,respectively.

Advantageous embodiments and further developments of the invention areevident from the respective subclaims.

In the method according to the invention and the device, the waterevaporating in the first stage of the method, with reaction componentscontained therein, is passed into the at least one further stage of themethod, and the expulsion of the water is effected only in the furtherstage or stages of the method.

In this way, the water can be separated off at relatively lowtemperatures and pressures and with reduced diamine losses.

The water evaporating in the first stage of the method, with reactioncomponents contained therein, is preferably passed via a rectificationmethod into the at least one further stage of the method.

A preferred technical solution for the rectification method comprises areflux column.

Of the greatest importance when using rectification methods in theoverall method is the use of reflux columns for those stages of themethod from which water is released into the environment. The loss ofreactive component is thus minimized to such an extent that there is nomarked influence on the final viscosity of the polyamide.

In the method according to the invention, an amount of the AH salt of upto 30% or from 80 to 100% is preferably used.

The expulsion of the water in the reflux column can be effected at atemperature of less than 120° C., caprolactam and diamine fractionsseparated off in the reflux column being recycled to the at least onefurther stage of the method.

The device according to the invention has a first pressure reactor,upstream of which a heat exchanger is connected and downstream of whichat least one postcondensation reactor is connected via a melt dryer, thegas space of the pressure reactor being connected with pressure controlto the gas space of the postcondensation reactor, and water from the gasspace of the postcondensation reactor is expelled via a reflux columnand a trap.

The caprolactam and diamine fractions separated off in the reflux columnare recycled to the postcondensation reactor.

The separation in the reflux column can be effected at a temperature ofless than 120° C.

The invention is explained in more detail below with reference toworking examples and to an embodiment of the device shown in thedrawing.

The drawing shows, in a schematic diagram, the arrangement according tothe invention of the stages of the method for the continuous productionof copolyamide.

A plant for the production of polyamide from aqueous AH salt withoutaddition of caprolactam, optionally also with additions of stabilizer,such as, for example, propionic acid, differs from the device accordingto the embodiment shown in this drawing only in that no lactam ismetered, the preheater is out of operation and there is the possibilityof controlling the pressure also after the reflux column.

WORKING EXAMPLE 1

In the embodiment shown in the drawing, the hexamethylenediamine salt ofadipic acid (AH salt) and lactam are reacted to give copolyamide.Aqueous AH salt and lactam are passed via, in each case, a preheater 1a, 1 b into a pressure reactor 2. Optionally, stabilizers and otheradditives are metered in after the preheating.

For a special product, 3100 g/h of an aqueous AH salt solution and 9800g of caprolactam/h are metered. On average, the weight ratio of AH saltto caprolactam was 20%. The system temperature in the pressure reactorwas 265° C.

The gas space of the pressure reactor 2 is connected via a reflux column7 and a pressure controller 10 to the gas space of a postcondensationreactor 5.

A pressure of 10 bar (gauge pressure) was set at the pressure controller10. A heating element 5.1 which promotes the expulsion of water isinstalled at the top of the postcondensation reactor 5, slightly belowthe product surface.

The reflux column 7 upstream of the pressure controller 10 is requiredfor ensuring that the reaction proceeds uniformly in the pressurereactor 2. The low-viscosity polymer prepared in the pressure reactor 2is passed into the postcondensation reactor 5 via a pump 3 with levelcontrol and via a melt dryer 4. The postcondensation reactor 5 can alsobe kept under slightly superatmospheric pressure up to 2 bar by means ofa further pressure controller 11. This is expedient in particular withthe use of a high percentage of 80% or more of AH salt in the startingmonomer and low target viscosity, as required as a starting material fortextile silk production.

In the melt dryer 4, the prepolymer is heated to 280° C., at the sametime the water present in excess being evaporated. This prepolymer islet down to atmospheric pressure in the postcondensation reactor 5, theprepolymer being passed via a devolatilization surface and the heatexchanger 5.1 present below the melt level, for better expulsion ofwater in vapor form. The excess water is removed from the process withpressure control via a reflux column 8 and a trap 9.

The product residence time in the postcondensation reactor 5 is, forexample, 5 hours. Separation of the gas mixture is effected in thereflux column 8. The ε-caprolactam runs back into the postcondensationreactor. The water leaves the polymerization process.

For expelling this water, hot nitrogen is passed into thepostcondensation reactor 5 so that water can readily diffuse out of thePA melt. This nitrogen and the water are removed from the process viathe trap 9. Heating of the nitrogen prior to entry into thepostcondensation reactor is particularly advantageous. This is not shownin the figure.

Copolyamide having an average melting point of 189° C. and a solutionviscosity, measured in 98% strength sulfuric acid, of 2.7 was produced.

The temperature level for the separation in the reflux column 8 can besubstantially reduced compared with known methods in which an additionalreflux column at a first pressure reactor is absolutely essential,namely to at least 100° C., with the result that diamine losses aresubstantially reduced.

The total extract after the postcondensation reactor 5 was 7.8%. Thematerial was then extracted and dried.

WORKING EXAMPLE 2

For the preparation of PA66, an aqueous AH salt solution was preheatedto >220° C. in the preheater 1 b, water also being evaporated in thepreheater. The system temperature in the pressure reactor 2 was 265° C.and the product residence time was 1 hour.

The polyamide 66 was fed into the postcondensation reactor 5 via thepump 3 and the melt dryer 4. The product residence time was 3 hours. Thepolyamide 66 was metered to the granulation process via the pump 6.

A pressure of 22 bar was set in the pressure reactor 2. Water and aminewere passed into the gas space of the postcondensation reactor 5 via thereflux column 7 and the pressure controller 10. Devolatilization waseffected from the postcondensation reactor 5 via the reflux column 8 andthe pressure controller 11 into the trap 9. 0.3 bar was set at thepressure controller 11. The gas space of the postcondensation reactorwas rendered inert with nitrogen. Polyamide 66 having a viscosity insolution viscosity units of 2.2 was prepared.

1. A method for the continuous production of copolyamide and polyamide,the starting material of which consists of the salt ofhexamethylenediamine with adipic acid (AH salt) and water and, in thecase of copolyamide, additionally of lactam, comprising a first stage ofthe method, which is carried out under pressure at temperatures between180 and 280° C., the prepolymer being fed, after passing through thefirst stage of the method, to at least one further stage of the method,from which the evaporated water is removed and/or is expelled with inertgas, characterized in that the water evaporating in the first stage ofthe method, with reaction components contained therein, is passed intothe at least one further stage of the method and the expulsion of thewater is effected only in the further stage or stages of the method. 2.The method as claimed in claim 1, characterized in that the waterevaporating in the first stage of the method, with reaction componentscontained therein, is passed into the at least one further stage of themethod, from which the waste product in vapor form is passed via areflux column outside the limit of the method and/or into the gas spaceof the next stage of the method.
 3. The method as claimed in claim 1,characterized in that an amount of AH-salt of up to 30% or from 80 to100% is used.
 4. The method as claimed in claim 1, characterized in thatthe expulsion of the water in the reflux column is effected at atemperature, at the upper end of the reflux column, of less than 120°C., and in that caprolactam and diamine fractions separated off in thereflux column are recycled to the at least one further stage of themethod.
 5. Device for carrying out the method as claimed in any of thepreceding claims, comprising a first pressure reactor (2), upstream ofwhich a heat exchanger (1) is connected and downstream of which at leastone postcondensation reactor (5) is connected via a melt dryer (4),characterized in that the gas space of the pressure reactor (2) isconnected with pressure control to the gas space of the postcondensationreactor (5), and in that water from the gas space of thepostcondensation reactor (5) is expelled via a reflux column (8) and atrap (9).
 6. The device as claimed in claim 5, characterized in that, inthe reflux column (8), the caprolactam and diamine fractions separatedoff are recycled to the postcondensation reactor (5).
 7. The device asclaimed in claim 5, characterized in that the separation in the refluxcolumn (8) is effected at a temperature of less than 120° C.